I'm in a union here in the UK. I think they're great.
I get 29 days holiday a year, and management tried to implement a policy to dictate that I had to take a block of two weeks leave every year. They claimed it was for health and safety reasons but I knew it was because it would save them time as they'd have to process less holiday requests. I took it to my union, they just laughed and shot it down immediately. They were also on hand when my contract was transferred between in employers and were helpful in talking me through the ins and outs and smaller details.
I suppose a union is only as good as the union rep you have. Some employers will bank on workers not knowing their rights etc and take advantage of that. A good union rep will know these things and be able to help you greatly if issues arise.